This invention relates to coatings of the type used to protect components exposed to high temperature environments, such as the hostile thermal environment of a gas turbine engine. More particularly, this invention is directed to the combination of a nickel-base superalloy substrate prone to the formation of deleterious reactions with aluminum-containing coatings, and a predominantly gamma-prime (γ′) phase nickel aluminide overlay coating that reduces the incidence of such reactions when used as an environmental coating or as a bond coat on the superalloy substrate.
Certain components of the turbine, combustor and augmentor sections that are susceptible to damage by oxidation and hot corrosion attack are typically protected by an environmental coating and optionally a thermal barrier coating (TBC), in which case the environmental coating is termed a bond coat that in combination with the TBC forms what may be termed a TBC system. Environmental coatings and TBC bond coats are often formed of an oxidation-resistant aluminum-containing alloy or intermetallic whose aluminum content provides for the slow growth of a strong adherent continuous aluminum oxide layer (alumina scale) at elevated temperatures. This thermally grown oxide (TGO) provides protection from oxidation and hot corrosion, and in the case of a bond coat promotes a chemical bond with the TBC. However, a thermal expansion mismatch exists between metallic bond coats, their alumina scale and the overlying ceramic TBC, and peeling stresses generated by this mismatch gradually increase over time to the point where TBC spallation can occur as a result of cracks that form at the interface between the bond coat and alumina scale or the interface between the alumina scale and TBC. More particularly, coating system performance and life have been determined to be dependent on factors that include stresses arising from the growth of the TGO on the bond coat, stresses due to the thermal expansion mismatch between the ceramic TBC and the metallic bond coat, the fracture resistance of the TGO interface (affected by segregation of impurities, roughness, oxide type and others), and time-dependent and time-independent plastic deformation of the bond coat that leads to rumpling of the bond coat/TGO interface. Therefore, advancements in TBC coating system are concerned with delaying the first instance of oxide spallation affected by the above factors.
Environmental coatings and TBC bond coats in wide use include alloys such as MCrAlX overlay coatings (where M is iron, cobalt and/or nickel, and X is yttrium or another rare earth element), and diffusion coatings that contain aluminum intermetallics, predominantly beta-phase nickel aluminide (β-NiAl) and platinum aluminides (PtAl). Because TBC life depends not only on the environmental resistance but also the strength of its bond coat, bond coats capable of exhibiting higher strength have also been developed, a notable example of which is beta-phase NiAl overlay coatings. In contrast to the aforementioned MCrAlX overlay coatings, which are metallic solid solutions containing intermetallic phases, the NiAl beta phase is an intermetallic compound that exists for nickel-aluminum compositions containing about 30 to about 60 atomic percent aluminum. Examples of beta-phase NiAl overlay coatings are disclosed in commonly-assigned U.S. Pat. Nos. 5,975,852 to Nagaraj et al., U.S. Pat. No. 6,153,313 to Rigney et al., U.S. Pat. No. 6,255,001 to Darolia, U.S. Pat. No. 6,291,084 to Darolia et al., and U.S. Pat. No. 6,620,524 to Pfaendtner at al. These NiAl compositions, which preferably contain a reactive element (such as zirconium and/or hafnium) and/or other alloying constituents (such as chromium), have been shown to improve the adhesion of a ceramic TBC, thereby increasing the spallation resistance of the TBC. The presence of reactive elements such as zirconium and hafnium in these beta-phase NiAl overlay coatings has been shown to improve environmental resistance as well as strengthen the coating, primarily by solid solution strengthening. However, beyond the solubility limits of the reactive elements, precipitates of a Heusler phase (Ni2AlZr (Hf, Ti, Ta)) can occur that can drastically lower the oxidation resistance of the coating.
The suitability of environmental coatings and TBC bond coats formed of NiAlPt to contain the gamma phase (γ-Ni) and gamma-prime phase (γ′-Ni3Al) has also been considered. For example, in work performed by Gleeson et al. at Iowa State University, Ni-22Al-30Pt compositions (by atomic percent; about Ni-6.4Al-63.5Pt by weight percent) were evaluated, with the conclusion that the addition of platinum to gamma+gamma prime coating alloys is beneficial to their oxidation resistance. It was further concluded that, because nickel-base superalloys typically have a gamma+gamma prime microstructure, there are benefits to coatings that also contain the gamma+gamma prime structure. Finally, Pt−containing gamma+gamma prime coatings modified to further contain reactive elements were also contemplated.
TBC systems and environmental coatings are being used in an increasing number of turbine applications (e.g., combustors, augmentors, turbine blades, turbine vanes, etc.). Notable substrate materials include directionally-solidified (DS) alloys such as René 142 and single-crystal (SX) alloys such as René N5. The oxidation and hot corrosion resistance of an environmental coating and the spallation resistance of a TBC deposited on a bond coat are complicated in part by the composition of the underlying superalloy and interdiffusion that occurs between the superalloy and the environmental coating or bond coat. For example, the above-noted coating materials contain relatively high amounts of aluminum relative to the superalloys they protect, while superalloys contain various elements that are not present or are present in relatively small amounts in these coatings. During coating deposition, a primary diffusion zone of chemical mixing occurs to some degree between the coating and the superalloy substrate as a result of the concentration gradients of the constituents.
To illustrate, FIG. 1 depicts an environmental coating 24 overlying a nickel-base superalloy substrate 22. As with many nickel-base superalloys, a primary diffusion zone 30 can be seen in the substrate 22 beneath the coating 24, as would be the case following a high temperature exposure. The primary diffusion zone 30 is represented as containing topologically close-packed (TCP) phases 32 in the gamma matrix phase 34 of the nickel-base superalloy substrate 22. The incidence of a moderate amount of TCP phases 32 beneath the coating 24 is typically not detrimental. At elevated temperatures, further interdiffusion occurs as a result of solid-state diffusion across the substrate/coating interface. This additional migration of elements across the substrate-coating interface can sufficiently alter the chemical composition and microstructure of both the coating 24 and the substrate 22 in the vicinity of the interface to have deleterious results. For example, migration of aluminum out of the coating reduces its oxidation resistance, while the accumulation of aluminum in the substrate 22 beneath the coating can result in the formation of a deleterious secondary reaction zone (SRZ) 36 beneath the primary diffusion zone 30. Certain high strength nickel-base superalloys that contain significant amounts of refractory elements, such as tungsten, tantalum, molybdenum, chromium, and particularly rhenium are prone to the formation of SRZ 36 containing needle-shaped precipitate phases 38 (such as γ phases and TCP phases of rhenium, tungsten and/or tantalum) in a gamma-prime matrix phase 40 (hence, characterized by a gamma/gamma-prime inversion relative to the substrate 22). Because the boundary between SRZ constituents and the original substrate 22 is a high angle boundary that doesn't tolerate deformation, the SRZ 36 and its boundaries readily crack under stress, drastically reducing the load-carrying capability of the alloy.
Notable examples of superalloys prone to deleterious SRZ formation include fourth generation single-crystal nickel-base superalloys disclosed in commonly-assigned U.S. Pat. Nos. 5,455,120 and 5,482,789, commercially known as René N6 and MX4, respectively. There have been ongoing efforts to develop coating systems and coating processes that substantially reduce or eliminate the formation of SRZ in high-refractory alloys coated with diffusion aluminide and overlay coatings. For example, ruthenium-containing diffusion barrier layers are disclosed in commonly-assigned U.S. Pat. No. 6,306,524 to Spitsberg et al. and commonly-assigned and co-pending U.S. patent application Ser. Nos. 09/681,821, 09/683,700, and 10/605,860 to Zhao et al. Even with such advancements, there remains a considerable and continuous effort to further improve the effectiveness of environmental coatings and TBC bond coats, while also mitigating any adverse affects these coatings may have on the substrates they protect.